1. Field of the Invention
The present invention relates to a developing device for use in an image forming apparatus of the electrophotographic type such as a copying machine and a printer, and more particularly to a developing device which eliminates harmful influences such as uniformed distribution due to stagnation of developer, increase of driving torque and stress to developer, by arranging to smoothly circulate the developer.
2. Related Art
As an example of a conventional developing device, there is a developing device described in Japanese Patent Laid-Open Application No. 5-333691. This developing device is constructed such that two upper and lower augers (hereinafter, referred to as "screw") 82, 85 are arranged in parallel so that their conveying directions are opposite to each other as shown in FIG. 30. While the upper and lower screws cause the developer to be given and received between both screws 82 and 85 as shown by arrows c and d and to be circulated, the developer is supplied to a developer carrier (developing roll) 87 provided adjacent the screw 82 to develop an electrostatic latent image on a photosensitive member through the developer carrier. In this developing device, the upper screw 82 supplies developer to the developer carrier 87 while the lower screw 85 collects the developer from the developer carrier 87.
In the conventional developing device described above, however, developer is not always circulated smoothly. This is because there is a defect in delivery of developer between two screws, or inclination is likely to occur in the distribution of developer within the developing device.
First, the delivery of developer will be described. The cause of the defect in the delivery is as follows. The conveying force of developer by each screw acts in the axial direction of the screw, and there is no conveying force in the delivery direction. Accordingly, setting aside the place (from above to below) indicated by the arrow c in FIG. 30 in which an external force, which is called gravity, acts in a direction to assist the delivery, gravity acts in a direction to hinder the delivery (carry up) conversely in the place (from below to above) indicated by the arrow d, and the efficiency of the delivery is noticeably low. In other words, in the place indicated by the arrow d, the developer is pushed out from back and overflows upwardly to thereby be only delivered (carried up).
For this reason, developer easily stagnates particularly in the place indicated by the arrow d, and it is necessary to noticeably increase the driving torque of the lower screw 85 in order to prevent such stagnation. Also, the stress may be applied to the developer stagnating in the place indicated by the arrow d to cause deterioration such as adhesion. Particularly, non-black color developer for use in a color image forming apparatus is more susceptible to stress than black developer, easily agglomerates, its deterioration is accelerated, or the after-treatment agent added to the developer may be removed. This has been a factor for hindering a vertical developing device from being adopted for a tandem image forming apparatus for which miniaturization is strongly requested. In this respect, a horizontal developing device with two screws horizontally arranged is the same in that the screws have no conveying force in the delivery direction, and therefore, has basically the similar problem in both delivery places although there is no hindrance due to gravity.
Next, the ununiformed distribution of developer will be described. A circulation path for developer in a developing device of this sort forms, as shown in FIG. 31, substantially a quadrilateral by the upper screw 82 (supply) and the lower screw 85 (collection). However, all the developer transferred from the downstream end 85b of the screw 85 to the upstream end 82a of the screw 82 does not reach the downstream end 82b of the screw 82, but part of the developer is supplied to the developing roll 87 midway, and passes through a developing nip to be collected in the course of the screw 85. The path of this part is indicated by arrows e in FIG. 31. The delivery of developer to the developing roll 87 is performed substantially over the entire width of the screws 82 and 85. For this reason, the amount of developer conveyed by the screw 82 gradually decreases from the upstream end 82a toward the downstream end 82b (FIG. 32). On the other hand, the amount of developer conveyed by the screw 85 gradually increases from the upstream end 85a toward the downstream end 85b (FIG. 33). This causes inclination in the distribution of the developer within the developing device.
Because of the inclination in the developer, the depth at which the screw 82 or 85 is actually immersed in developer is not uniform. More specifically, the screw 82 is more deeply immersed in developer toward the upstream end while the screw 85 is more deeply immersed in developer toward the downstream end. Therefore, FIGS. 32 and 33 represent the depth of the developer. Such ununiformity in depth of developer causes ununiformity in the delivery of developer between the screws 82, 85 and the developing roll 87, and as a result, the density of an image formed differs depending on the width direction (axial direction) of the screws 82, 85.
As the measure, it is possible to relatively reduce the ununiformity by increasing the conveying speed of the screws 82, 85, or to lighten the ununiformity by providing a third screw between the screw and the developing roll 87 as disclosed in Japanese Patent Laid-Open Application No. 6-51634. In the case of relatively reducing the ununiformity by increasing the conveying speed of the screws 82, 85, however, it is necessary to further increase the conveying speed when the degree of the ununiformity is great. Therefore, it is necessary to control so as to increase the conveying speed in response to the amount of developer which passes through the developing nip. Since there is a limit to the increase in the conveying speed, it functions effectively only in an area where a small amount of developer passes through the developing nip. Also, in the case of reducing the ununiformity by providing a third screw between the screw and the developing roll 87, the structure of the developing device becomes complicated, and the number of parts also increases, causing the cost to be increased.
The present invention has been achieved in order to solve the above-described problem of the conventional developing device. The object is to provide a developing device which makes the delivery of developer within the developing device smooth, and lightens the ununiformed distribution of the developer.